A fuel cell is a device that uses hydrogen (or hydrogen-rich fuel) and oxygen to create electricity by an electrochemical process.
A single fuel cell consists of an electrolyte sandwiched between two thin electrodes (a porous anode and cathode). There are different fuel cell types, for all hydrogen, or a hydrogen-rich fuel, is fed to the anode, oxygen, or air, to the cathode.
For instance, for polymer exchange membrane (PEM) and phosphoric acid fuel cells, protons, generated by the anode reaction, move through the electrolyte to the cathode to combine with oxygen and electrons, producing water and heat.
For alkaline, molten carbonate, and solid oxide fuel cells, negative ions travel through the electrolyte to the anode where they combine with hydrogen to generate water and electrons. The electrons from the anode side of the cell cannot pass through the membrane to the positively charged cathode; they must travel around it via an electrical circuit to reach the other side of the cell. This movement of electrons is an electrical current.
Molten Carbonate Fuel Cells (MCFC) are in the class of high-temperature fuel cells. The higher operating temperature allows them to use natural gas directly without the need for a fuel processor and have also been used with low-Btu fuel gas from industrial processes other sources and fuels. Developed in the mid 1960s, improvements have been made in fabrication methods, performance and endurance.
MCFCs work quite differently from other fuel cells. These cells use an electrolyte composed of a molten mixture of carbonate salts. Two mixtures are currently used: lithium carbonate and potassium carbonate, or lithium carbonate and sodium carbonate. To melt the carbonate salts and achieve high ion mobility through the electrolyte, MCFCs operate at high temperatures (650° C.).
When heated to a temperature of around 650° C., these salts melt and become conductive to carbonate ions (CO32−). In operation, these ions are generated by the cathode reaction and flow from the cathode to the anode where they combine with hydrogen to give water, carbon dioxide and electrons. These electrons are routed through an external circuit back to the cathode, generating electricity and by-product heat.
The reactions which take place are the following:Anode Reaction: CO32−+H2=>H2O+CO2+2e−Cathode Reaction: CO2+½O2+2e−=>CO32−Overall Cell Reaction: H2(g)+½O2(g)+CO2 (cathode)=>H2O (g)+CO2 (anode)
The higher operating temperature of MCFCs has both advantages and disadvantages compared to the lower temperature PAFC and PEFC. At the higher operating temperature, CO is a fuel and not a poison, with formidable benefit in terms both of fuel source acceptability and of methane reforming. Moreover, with regard to the reforming of natural gas, it can occur by using directly as thermal source the waste heat itself of the MCFC. Additional advantages include the ability to use standard materials for construction, such as stainless steel sheet, and allow use of nickel-based catalysts on the electrodes. The by-product heat from an MCFC can be used to generate high-pressure steam that can be used in many industrial and commercial applications.
With regard to the main functional requirements, the complete filling of every interstice in the porous ceramic component inserted between anode and cathode (electrolyte matrix) is a key factor. This not only constitutes a barrier for the gases but also prevents the sinking of performance (in fact, in the vacancies of electrolyte the transport to the anode of the CO32− ion generated by the cathode does not occur). For both the electrodes, an essential characteristic for the completion of the reactions is the constant presence of a triphasic contact surface gas/C32− ion/electron. It is then evident that besides the electro-catalytic properties of the electrode material, the performance depends on the access/removal from the reaction sites of gas and CO32− ion. The performance is therefore unacceptable as well when the electrodes are “flooded” as when the electrolyte quantity is insufficient.
For each of the two electrodes there is an optimal filling degree for enhancing the performances; moving away from it means diminishing the efficiency degree. There is anyway a range in which such degree is still acceptable and the cells characteristics can be in any case exploited.
In working condition, since the carbonates amount gradually decreases, the filling degree changes with the time. In order to maintain the filling levels higher then the minimum required, it is necessary to start from filling levels which are higher than the optimal ones.
The carbonates distribute spontaneously in the pores with higher capillary retention properties.
In order to control the electrolyte repartition among components, it is necessary to choose the correct volumes in the design of the cell and respect a rigorous hierarchy on the base of their retention properties.
Since it is necessary that the matrix remains in any case full, the diversion of the filling degree must regard exclusively the electrodes. The reaction at the anode is less penalized than the one which takes place at the cathode by the filling degrees which are far away from the optimal values. Hence, the design of the cell tends therefore to use the anode by concentrating on it the initial surplus of electrolyte and evacuating it for first. The configuration of a single cell consists of a 3-layers “sandwich” (anode, matrix of electrolyte, cathode) placed between metallic pieces intended for the distribution of gas and for the transport of current. In practice, a molten carbonate fuel cell (which conventionally is indicated with the word “stack”) is a modular structure constituted by many elementary cells electrically connected in series but with parallel gas inlets. The electrical connection between the cells is achieved with a metallic separator (electronic conductor) between the anode compartment of the one cell and the cathode compartment of the adjacent one. At each end of the cell there is a metallic plate; they are normally indicated as “end plates”, one at the anode and the other at the cathode.
In order to provide the single cells with gas, the commonly used ways are two: external manifold and internal manifold. In the first case each one of the four lateral faces has a specific function: fuel inlet, fuel outlet, oxidation agent inlet, oxidation agent outlet. The first and the second couple of faces are in opposite positions to each other. Normally, on every face a manifold is placed directly against the face of the cells pack. From the internal area to the manifold the gas reaches directly the parts to which it is designated.
In the internal manifold, the inlet conducts of the gas to the single cells are obtained through complex grooves in the bipolar plates which divide one cell from the other.